Determination of slow slip episodes and strain accumulation along the Cascadia margin

Abstract

[1] Continuous GPS stations in the Pacific Northwest Geodetic Array network clearly record subduction-related strain accumulation and slow slip episodes along the Cascadia convergent margin. Many of the slow slip episodes have been correlated in time and space with seismic evidence for nonvolcanic tremor, leading to the previous discovery of episodic tremor and slip (ETS). In this study, we use a hyperbolic tangent curve fitting technique for the identification of slow slip times and displacement magnitudes within the GPS time series, independent of seismic tremor data. We then apply this technique to study the patterns of strain accumulation and release associated with ETS events and characterize patterns of coupling associated with the locked and transition zones of the plate interface. We demonstrate the effectiveness of this automated technique for both identification of slow slip observations and calculation of slow slip displacements. Recurrence patterns in the distribution of GPS observations demonstrate coherence among neighboring stations over time and apparent along-strike segmentation of the subduction interface. When slow slip events are removed from the time series, we can estimate the total site velocities between slow slip events. These velocities decay as depth to the subduction interface increases, but they diverge from the long-term trends expected from the interseismic cycle at $30-60 km above the interface, consistent with the location where slow slip displacements occur. Forward modeling of coupling on the plate interface reveals that in between slow slip events there is a patch of at least 30% coupling from 20 to 35 km depth, which is needed to produce the observed back slip displacements. Intriguingly, our best fitting models have a decrease in coupling down to $30% at $20 km depth followed by a peak of greater than 80% coupling at $30-35 km depth, suggesting the source zone for ETS events acts as a distinct locking zone that releases strain more frequently than the updip seismogenic locked zone, although a zone of constant $30% coupling cannot be ruled out with this data set. Such a scenario indicates that frictional behavior with depth follows a more complex model than a simple temperature controlled transition. We propose that coupling initially decreases with depth due to a decrease in strength of the overriding lower crust, but then coupling increases again when the subducting plate comes in contact with the stronger overriding mantle.Citation: Holtkamp, S., and M. R. Brudzinski (2010), Determination of slow slip episodes and strain accumulation along the Cascadia margin,

The Cascadia Initiative: A Sea Change In Seismological Studies of Subduction Zones

“…GPS and leveling measurements document northeastward compression above the megathrust, indicating that it is presently locked and accumulating interseismic strain and is in the late (high stress and low strain rate) stage of the subduction cycle (Dragert et al, 1994;Mitchell et al, 1995;Wang, 2003;Burgette et al, 2009;Chapman and Melbourne, 2009;Holtkamp and Brudzinski, 2010;McCaffrey et al, 2012McCaffrey et al, , 2013. Despite the accumulating strain and the history of great earthquakes, the current seismicity is largely confined to intermediate depth, in-slab events in the downgoing Juan de Fuca Plate, largely beneath the Puget Lowland and around the Mendocino Triple Junction, and eerily is almost nonexistent on the plate boundary (Wells et al, 1998;McCrory et al, 2012).…”

Characteristics and interactions between non-volcanic tremor and related slow earthquakes in the Nankai subduction zone, southwest Japan

“…From north to south, the recurrence interval in each zone is 14, 19, and 9 months. The zones are divided into seven large segments with along-strike lengths of 100-200 km, according to the discontinuities in seismic tremor activity and GPS displacements (Brudzinski and Allen, 2007;Holtkamp and Brudzinski, 2010). In Cascadia, ETS events much smaller than other well-known ETS events, in terms of both size and duration, have been detected by GPS and borehole strainmeters .…”

Rheology and stress in subduction zones around the aseismic/seismic transition

“…The character and properties of the creeping zone are poorly known: many analyses assume that it is simply a zone of stable slip along the interface (e.g., Holtkamp and Brudzinski 2010), but exhumed rocks from these depths suggest there may be a so-called subduction channel containing metasedimentary and volcanic rocks, which take up much or all of the displacement (Gerya 2002;Warren et al 2008;Beaumont et al 2009;Blanco-Quintero et al 2011;Behr and Platt 2013). The transition between the seismic and aseismic zones is particularly interesting, as in a number of subduction zones this is the source of tectonic tremor and slow slip events (Hirose et al 1999;Dragert et al 2001;Obara 2002), the origin of which is uncertain (Fig.…”